The present invention relates to a fiber preform for producing fiber composite structures or composite components, the wall thereof being made of reinforcing fibers, as well as to a composite component made from this type of fiber preform.
Components made from fiber composite materials are increasingly used, especially in the aerospace industrial sectors, yet also e.g. in machine building industry. Fiber composites often offer the advantage of lower weight and/or higher strength over metals. An essential aspect thereby is the inexpensive production of this type of resilient and yet light-weight composite components at the same time. In view of the resilience, i.e. the rigidity and strength, the volume percent of the reinforcing fibers and especially also the direction of the reinforcing fibers have a determining effect on composite components.
A commonly used manufacturing method is currently based on the so-called prepreg technology. In this case, the reinforcing fibers, such as glass fibers or carbon fibers, are arranged for example parallel to one another, embedded in a matrix resin, and processed into sheet-like semi-finished products. For component manufacture, these sheets are cut according to the component contour and laminated into a tool by machine or by hand layer-by-layer while taking into account the orientation of the reinforcing fibers as required by the component load. Subsequently, the matrix is cured under pressure and at temperature in an autoclave. This type of manufacturing process is, however, very complex and expensive for many components.
In a further method, so-called fiber preforms are produced from reinforcing fibers. Essentially, these are textile semi-finished products in the shape of two- or three-dimensional configurations made from reinforcing fibers, in which, in further steps for producing the fiber composite component, a suitable matrix material is introduced via infusion or injection, also by application of vacuum. Subsequently, the matrix material is cured at, as a rule, increased temperatures and pressures into the finished component. Known methods for infusion or injection of the matrix material in this case are the so-called liquid molding (LM) method, or methods related thereto such as resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), resin film infusion (RFI), liquid resin infusion (LRI), or resin infusion flexible tooling (RIFT). The fiber material used to produce the fiber preforms can also already be impregnated e.g. with low amounts of a curable plastic material, i.e. a binder material, in order to improve the fixing of the reinforcing fibers in the fiber preform. Pre-impregnated yarns of this type are described for example in WO 2005/095080.
In order to produce such fiber preforms, WO 98/22644 has already suggested dispersing short-cut reinforcing fibers together with a binder material on an air-permeable screen adapted to the shape of the desired fiber preform and maintaining said fibers on the screen through the application of vacuum until, after cooling of the binder material, a sufficient stability of the preform is achieved. By means of this procedure, the reinforcing fibers are arranged in random, isotropic arrangements and directions. This is indeed advantageous if the load directions in the component cannot be predicted in advance; however, it has the simultaneous disadvantage that, due to the isotropic orientation, only a fraction of the fibers lie in the load direction. An adaptation to special load directions in the component is thus not possible when using this method. Reinforcements in the component wall can, at most, be made via e.g. locally increased wall thicknesses; however they are associated with an increase in weight of the component. In addition, according to the examples of WO 98/22644, only fiber volume proportions in the range of up to approximately 15 vol. % are achieved, and therefore, due to the low fiber volume proportions, only comparably low thickness-related component strengths. Usually, fiber proportions of a maximum of 30 vol. % are achieved for components of this type having random orientation of the reinforcing fibers.
In US 2010/0126652 A1 and US 2009/0229761 A1, a method and a device, respectively, for producing fiber preforms are described, by means of which it is possible to satisfy the demand for a load-appropriate fiber direction in the component. In this case, a so-called TFP method (“tailored fiber placement method”) is used, in which yarns or fiber strands are laid along any number of paths adapted to the distribution of forces affecting the finished component and pre-fixed using fixing threads, wherein CNC controlled sewing and knitting machines are used therefor. US 2009/0229760 A1 describes an application device for the fiber strands suitable for a TFP method of this type. Using these TFP methods, an improved utilization of the mechanical resistance of the reinforcing fibers and an increased adaptation of the component cross sections to the respective local loads in the component are possible. However, these methods, in particular in the production of fiber preforms with complex, three dimensional structures, are complex and cost intensive.
As an alternative to fixing the fiber strands by means of a textile method, such as by means of sewing or knitting methods, the fiber strands can also be fixed by means of a thermally activated binder material, for example by means of a thermoplastic, as is described in DE 10 2007 012 608 B4.
A further possibility for the production of fiber preforms consists in the use of so-called multiaxial non-crimp fabrics. Multiaxial non-crimp fabrics are understood to be structures made from a plurality of superimposed fiber layers, wherein the fiber layers comprise sheets of reinforcing yarns arranged parallel to each another. The superimposed fiber layers can be connected and secured to each other via a plurality of sewing or knitting threads arranged side-by-side and running parallel to each other and forming stitches, such that the multiaxial non-crimp fabrics is stabilized in this way. The fiber layers are superimposed such that the reinforcing fibers of the layers are directed parallel to each other or alternately crosswise (e.g. −45°; 0°, +45°.
Multiaxial non-crimp fabrics of this type are laid without matrix material in a mold and e.g. for shaping are adapted to its contours using increased temperature. Subsequently, the matrix material required for the production of the composite component is introduced into the mold and into the fiber preform via infusion or injection, whereby, following curing of the matrix material, the composite component is obtained. Multiaxial non-crimp fabrics and the use thereof to produce fiber preforms are described for example in EP 0 361 796 B1, EP 1 352 118 B1, or WO 98/10128.
Multiaxial non-crimp fabrics are, however, expensive to produce, and are generally produced in standard widths, which seldom correspond to the dimensions of the later component. This results in a not insignificant amount of waste. In addition, especially in components with complex contours and particularly with respect to components with small radii of curvature, they can only be used to a limited extent, as the multiaxial non-crimp fabrics cannot be draped into any form. Further, it was observed that the sewing or knitting threads can often lead to a reduction in the impact strength of the resulting composite. Finally, the later infusion or injection of the matrix material can also be slowed down over the liquid molding or related methods.
To avoid seams and transverse filaments, US 2008/0085650 A1 suggests using reinforcing material structures having a layered construction, said reinforcing material structures comprising a layer of continuous reinforcing fibers directed in parallel as well as a layer made from e.g. a non-woven, a woven fabric, or from short cut fibers, wherein the layers are connected to each other via an adhesive or via adhesive points. These materials are also initially available in standard widths, which have to be cut corresponding to the component geometry. In this way, increased costs occur due to additional steps, for example cutting, draping, and connecting, as well as an average waste of up to 30% of the output material.